It is known to use glass or metal fibers, as a mesh, woven cloth or wool, as a catalyst support. The catalyst is conventionally applied to the support by simply impregnating it with a solution of the catalytically active material, or a precursor thereof, followed by drying and/or activation by calcining. For instance, in U.S. Pat. No. 4,200,609, a catalyst impregnated fiber mesh (made from, for instance, a glass fiber substrate) is prepared by contacting the substrate with a solution of permanganate in water, and the resulting catalytic structure is used in the form of conical fibric bags or filter panels placed in air ducts for the removal of ozone, for instance from the air supply to aircraft cabins.
A high temperature catalyst system particularly adapted for a catalytic converter is proposed in U.S. PAT. No. 3,189,563 wherein the catalyst is formed on a glass fabric carrier. An open mesh glass fabric supported catalyst is prepared by applying a fluid dispersion or slurry of refractory ceramic precursors to a glass fabric, which is thereafter calcined to convert the coating to the refractory material. At the high temperatures employed, the hardened refractory mass essentially becomes the support in place of the glass fibers, and preserves the original open mesh design of the fabric. The calcined structure preserves the original open mesh design of the fabric. The calcined product is described as a large pore carrier of refractory material, having a skeleton of more or less disintegrated threads, which may be coated or impregnated with the catalytic materials by conventional methods. The refractory materials thus prepared are said to resist heat shock very well, but the flexibility of the glass fiber is lost, and the fibers are easily broken by mechanical or physical shock.
To overcome the brittleness experienced with the ceramic support of the above '563 patent, U.S. Pat. No. 4,038,214 proposes using a woven high silica glass fiber, rather than ceramic coatings, to obtain greater strength and heat resistance. The woven glass fiber is impregnated with a solution of the catalyst by conventional techniques, and activated by calcining at a high temperature.
It is also known that cloth or fibers which have been treated to be chemically, physically or chemical-physically active can be used in filter bags for the simultaneous removal of particulates and noxious gases from gas streams such as flue gas. For instance, it has been proposed in U.S. Pat. Nos. 4,220,633 and 4,309,386 to use catalytic filter bags made of a glass, metal, refractory or ceramic fabric in a filter house structure to simultaneously remove entrained particulates and nitrogen oxides from flue gases. The NO.sub.x removal is accomplished by injecting ammonia into the flue gas and passing it through filter bags in which an appropriate catalyst has been incorporated, for instance as drawn filaments interspersed with the filter fabric fibers. Alternatively, it is suggested that the filter bags may be coated with the catalyst by spraying the fibers with a suspension of finely divided catalyst, or with a solution of the appropriate metal catalyst salts which may be subsequently converted to the desired oxide form, or by applying a finely divided catalyst to the filter bags as a pre-coat prior to placing them in service.
Although a glass fiber cloth or mesh substrate already provides a somewhat enhanced surface area relative to a solid or sheet material, such substrates coated with catalytic materials by conventional techniques still have only a limited catalytic activity, particularly at the higher space velocities encountered, for instance, in filter bag applications. Moreover, applications such as filter bags require that the catalytically active fabric material be not only self-supporting, but flexible and highly resistant to abrasion and/or self-abrasion, and capable of withstanding elevated temperatures. None of the above known materials or processes adequately satisfy this combination of requirements.
It has now been found that a highly porous, adherent and abrasion resistant coating of one or more catalytically active metal oxides may be applied to a fibrous substrate, while still maintaining flexiblity, by coating the substrate with a metal alkoxide precursor of the desired metal oxide, in accordance with an improved and particularly adapted sol-gel process.
The sol-gel process has typically been utilized in the past to produce extremely homogeneous, low temperature oxide glasses, starting with metal alkoxides. Most commonly, in the polymerization type of process, the metal alkoxides are reacted as a unit to form alkoxide complexes that are later subjected to controlled hydrolysis reactions. Thus, the metal alkoxides are dissolved in the desired proportions in an organic solvent, such as methanol or ethanol. If a less reactive alkoxide is used, such as silicon alkoxide, it may be desirable to reflux the solution for several hours under an inert atmosphere to promote the complex formation. The solution is then permitted to react with mositure present in the atmosphere, or added in controlled amounts, to promote the hydrolysis-condensation reactions. The hydrolysis reaction results in the replacement of organic groups by hydroxyl groups and, in the condensation reaction, hydroxyl groups condense by splitting off water, resulting in a network structure or gel. The gel thus formed, which consists of an open oxide network containing unreacted alkoxide groups, hydroxy groups, solvent, and reaction products, is dried and heated to temperatures of from about 250.degree. C. to about 500.degree. C. in order to remove residual organic and hydroxyl groups. The dried gels may then be further heated to a temperature sufficient to eliminate porosity and consolidate the structure.
In more recent years, techniques have been developed whereby the gel thus produced can be applied as a coating to substrates, particularly large panes of glass, to obtain desired optical properties. Thus, in a dip-coating procedure described H. Dislich and E. Hussmann, Thin Solid Films, Vol. 77, No. 129 (1981), large glass panes are carefully cleaned, dipped into a solution containing hydrolyzable metal compounds and then pulled out at a constant speed into an atmosphere containing water vapor. In this atmosphere, the hydrolysis and condensation reactions take place until transparent metal oxide layers are obtained. Finally, the film is hardened in a high temperature cycle until a transparent metal oxide film has been formed. Such sol gel coating techniques have heretofore been directed primarily toward applying optical coatings to planar surfaces such as glass plates or sheets.